Commonly, plate members having a plurality of through-holes, such as a tube plate and a tube support plate of a condenser, are drilled with a machine tool on which a drilling tool such as a high-speed steel drill or a cemented carbide drill is mounted. During drilling, typically, the drilling tool is inserted into a plate member while the outer peripheral part of the plate member is fixed with a clamp. At this point, if the rigidity around the drilling position is low, the plate member becomes bent due to insertion of the drilling tool or the plate member vibrates due to rotation of the drilling tool, which may degrades the machining accuracy of through-holes, and may further cause breakage of the cutting edge of the drilling tool. Therefore, it is required to fix the plate member so as to provide the plate member with sufficient rigidity to withstand drilling.
There is a known method in which, in order to enhance the rigidity of a plate member, a dummy plate is laid on the lower surface of the plate member during drilling, and the plate member and the dummy plate are drilled together with a drilling tool. In this case, since the dummy plate provides the plate member with a reaction force during insertion of the machining tool, significant bending of the plate member can be prevented. However, since the drilling tool rotating at a high speed may cause the plate member to bounce and break the drilling tool, it is necessary to ease the machining conditions of the drilling tool (tool rotational speed, tool feeding speed). Moreover, it takes time and effort to remove foreign matters from or take care of the dummy plate due to its large area of contact with a plate member, and since the dummy plate needs to be prepared for each of plate members having different hole arrays, storage cost is incurred.
A fixing method not using a dummy plate is disclosed in Japanese Patent Publication No. H11-320223, for example, in which spacers are interposed between each adjacent ones of a plurality of plate members stacked on a receiving cleat of a surface plate, and the outer peripheral part of the plate members is fixed with clamps. Japanese Patent Publication No. H5-146905 discloses a method in which the outer periphery of a plate member is fixed with clamps, and the plate member is supported from the drilling tool side by means of cylinders which move on rails. Japanese Utility Model Publication No. S64-12711 discloses a configuration in which a thrust force absorbing member made of a square bar, etc. is provided between a plate member and a bed so as to absorb the thrust force of a drill and vibration during boring. In this configuration, a pressing member which presses the plate member near a part to be bored is further provided to thereby prevent breakage of the drill due to bouncing of the plate member.